1. Field of the Invention
The present invention relates to a process for the production of aqueous dispersions containing polyols and blocked polyisocyanate crosslinking agents, which dry as a powder on the surface to be coated and, after stoving, yield coatings with improved water and solvent resistance.
2. Description of the Prior Art
Aqueous polyurethane dispersions are known (c.f. Houben-Weyl, Methoden der organ. Chemie, 4th edition, volume E 20, page 1659 (1987)). However, where higher quality is demanded, for example, in automotive lacquer coatings, non-reactive polyurethanes exhibit various weaknesses. The essential reason is the lack of crosslinking between the film-forming macromolecules, which results in diminished water and solvent resistance and poor mechanical properties. More recent post-crosslinking coating systems, such as those obtained by combining isocyanate-reactive resins with blocked polyisocyanate crosslinking agents (c.f. J. W. Rosthauser, K. Nachtkamp in Advances in Urethane Science and Technology, K. C. Frisch and D. Klempner, editors, volume 10, pages 121-162 (1987)), provide better properties.
The resins used are polyurethane, polyepoxy, polyester or polyacrylate resins or dispersions which are crosslinkable via hydroxyl groups. The crosslinking agents are blocked polyisocyanates, which have optionally been hydrophilically modified. Such systems are known, for example, from DE-OS 4,213,527, EP-A 581,211, EP-A 427,028, U.S. Pat. No. 4,543,144, DE-OS 3,345,448 and DE-OS 2,829,648.
Systems which may be considered for lacquer and coating applications have the characteristic of exhibiting good film-forming properties even at room temperature. If this characteristic is not present, films having overall poorer properties are obtained, e.g., poorer levelling and lower gloss. Film formation is sometimes improved by the addition of solvents.
The application of powder coatings from the aqueous phase is described, for example, in EP-A 652,264. A disadvantage is that, during production, extrusion of the binder is followed by a grinding operation, which is very elaborate and associated with high costs. Moreover, there are limits to powder fineness in a grinding operation.
It has now been found that it is possible to economically obtain valuable, solvent-free, heat curing lacquer dispersions which dry as a powder by combining certain isocyanate-reactive resins with blocked polyisocyanate crosslinking agents. The resulting products are valuable coating compositions, which may be applied as a one-component system to provide coatings having particularly high quality surface properties. It should be emphasized that highly level, high gloss coatings having very good water and solvent resistance are obtained.
A further advantage is that even though the binders dry as a powder, coatings produced according to the invention may be applied using existing liquid lacquer equipment. Thinner coats are obtained than with conventional powder coating, and cleaning operations are simplified in comparison with conventional powder coating since the equipment and booths may be spray cleaned. Cleaning operations are also simplified in comparison with solvent-based lacquer coating because neither film formation nor, in comparison with two-component lacquers, crosslinking occurs at room temperature.
Known conventional aqueous coating compositions, which form coatings at temperatures as low as room temperature, often have only a narrow application window (range of temperature and relative atmospheric humidity during application) and have a distinct tendency to blister (pinholing). Eliminating these difficulties is distinctly more favorable with binders which dry as a powder. Surface properties are less dependent upon climatic conditions (temperature, relative atmospheric humidity) during application. Greater film thicknesses may also be achieved without pinholing due to evaporation of the water.